A capacitive pressure sensor, particularly a capacitive microphone, configured as a pressure difference sensor, generally comprises a very thin metallized diaphragm that forms one plate electrode of a capacitor and a back plate forming a second plate electrode of the capacitor. The diaphragm is tightly stretched to have a high resonant frequency, and is placed at a distance very close to the back plate. The diaphragm is deflected by a pressure difference between its front surface and its back surface, wherein the back surface of the diaphragm together with a housing of the pressure sensor encircles a back volume and seal it against the environment. Deflecting the diaphragm causes slight changes of the capacitance of the capacitor. The back volume constitutes a reference pressure which is decisive for the function of each capacitive pressure sensor. Large back volumes are required for achieving high sensitivities of capacitive pressure sensors, since large back volumes enable larger deflections of the diaphragm in reaction to pressure differences between its front surface and its back surface. However, large back volumes are very disadvantageous for micro-electro-mechanical-system (MEMS) based sensors, since they dramatically limit potential miniaturization and cause higher production costs.
One approach to overcome the requirement of large back volumes for high-sensitive capacitive pressure sensors is to evacuate the back volume. Then the back volume has to be just as large as the volume between the diaphragm and the back plate. See e.g. US 2006/0230835. The disadvantage of this method is that the diaphragm has to biased with the environmental pressure being about five orders of magnitudes higher than the average sound pressure for microphones. US 2006/0230835 suggests to circumvent this disadvantage by applying a high bias-voltage across the diaphragm and the back plate which bias-voltage urges the diaphragm against the return force exerted by the pressure difference into an optimum distance from the back plate. Nevertheless, with this concept, the diaphragm is always in a critical state, since even smallest variations of the bias-voltage and/or the environmental pressure, respectively, cause the diaphragm to collapse against the back plate. This collapse inevitably occurs when the diaphragm is deflected to a position where electrostatic force overcomes the mechanical restoring force of the diaphragm. The reason for this behaviour is the reciprocal relation of the electrostatic force and the distance between the diaphragm and its back plate, i.e. that the electrostatic force increases with a decreasing distance between diaphragm and back plate. Hence, if the actual distance between the diaphragm and its back plate falls below a critical distance which depends on the mechanical dimensions and the bias-voltage then the diaphragm cannot maintain a stable location and collapses against the back plate. Particularly for capacitive microphones the pressure differences caused by sound waves to be measured by the microphone are considerably smaller than variations in the environmental pressure caused by weather changes or varying altitudes. Therefore, for capacitive microphones the actual bias voltage has to be set far away from an ideal operating point that would be reached when the diaphragm is held quite close to the critical distance to its back plate. The result of these necessary engineering compromises are microphones with reduced sensitivities.